In order to ensure greater purification the sewage in the subsiding tanks is now usually mixed with certain chemical reagents, which, it is believed, have the effect not only of speedily precipitating the solid materials, but also carrying down injurious matters suspended in the sewage water, thus rendering it sufficiently pure to be discharged without risk to health into any watercourse.
Of the numerous precipitants employed for this purpose, we may mention the following:
Lime and salts of lime. Quicklime, in the proportion of 8 gr. to a gallon of water; or 1 lb. to about 600 galls. of sewage; lime, with the addition of about a fortieth of its weight of chloride of lime; calcic phosphate dissolved in sulphuric acid; Whitehead’s patent, which consists of a mixture of mono- and dicalcic phosphate; chloride of calcium.
Aluminous compounds. Bird’s process—A mixture of aluminous earths and sulphuric acid. Andersons and Lenk’s—Impure sulphate of alum; refuse of alum works, either alone or mixed with lime or charcoal. Scott’s cement process—Clay mixed with lime; natural phosphate of aluminium dissolved by sulphuric acid and mixed with lime.
The quantities of the above substances when used as precipitants vary, in some of them fifty, and in others eighty grains to a gallon of sewer water being employed.
Magnesium salts. Impure chloride of magnesium mixed with superphosphate of lime.
Carbon. As vegetable charcoal, peat, seaweed charcoal, carbonised tan, lignite, and Boghead coke.
Iron. In the form of sulphate. Ellerman’s and Dale’s—Perchloride; the sulphate is sometimes mixed with coal dust.
Manganese. Condy’s fluid.
Zinc. As sulphate and chloride.